You could either have a very slow leak where the coolant is evaporating before it can leak onto the ground or there is a leak into the cylinders. Look very carefully around all of the hose connections, the water pump area, and the radiator for coolant residue. The residue will be a white or redish powdery substance. The residue could indicate a very slow coolant leak. Also, pressurizing the cooling system may help you find your leak.

Many of these engines had cylinder heads made by a company called castek. They are known for cracking around the head bolts and coolant slowly seeps into the engine oil and is absorbed and boiled away by engine heat. Here's more information: http://www.agcoauto.com/content/news/p2_articleid/148